DNA damage by solar ultraviolet (UV) radiation is the major cause of skin cancer, the most common type of cancer in the US. A cellular "UV response" involving activation of the p53 tumor suppressor protein has evolved to protect the genome against this solar threat. An intact response leads to cell cycle arrest and allows DNA to be repaired prior to replication, preventing the permanent incorporation of mutations. Roughly 90% of squamous cell carcinomas lack this UV response indicating the essential function of this pathway in protecting the genome from UV carcinogenesis. Despite the importance of this protective pathway, it is not known how DNA damage by UV leads to p53 induction. A gene called ATR has been cloned in the Schreiber group and appears likely to be the mediator of the UV-p53 response based on preliminary data. ATR belongs to a newly described family of proteins called the PIK-related kinases (phosphatidyl inositol kinase-related kinases), which includes the gene mutated in the recessive lethal disease, ataxia telangiectasia. The PIK-related kinases mediate cell cycle arrest after cellular stresses such as DNA damage. Molecular and biochemical approaches will be used to test the hypothesis that ATR or another PIK-related kinase is required for the UV response. Endogenous ATR function will be inhibited with a dominant negative ATR allele expressed in an inducible or retroviral system. The role of ATR will then be examined in individual aspects of the response to UV-DNA damage. Later studies will focus on other proteins in this pathway including putative substrates for the UV-responsive protein. This project aims to provide insight into the molecular mechanism of UV carcinogenesis and may suggest approaches for the prevention and treatment of skin cancer and cancer more generally.